Laser shock peening (also known as the LaserPeen® process, laser shock processing, or laser peening) is an innovative surface treatment for improving the fatigue strength and damage tolerance of metal parts. Laser shock peening drives high amplitude shock waves into a material surface using high intensity laser pulses. The shock waves are used to develop deep compressive residual stresses in the surfaces of fatigue-prone parts. Typically, these stresses penetrate five to ten times deeper than conventional metal shot peening. These compressive surface stresses inhibit the initiation and propagation of fatigue cracks.
Before processing, an overlay coating, which may be substantially opaque to the laser beam, may be applied to the material surface being treated. An additional layer, which may be substantially transparent to the laser beam, may be placed over the opaque overlay. The opaque overlay may be, for example, black paint or tape. The transparent overlay may be, for example, flowing water.
The laser pulses pass through the transparent overlay and strike the opaque overlay, causing the opaque overlay to vaporize. The vapor absorbs the remaining laser energy and produces a rapidly expanding plasma plume. Since the expanding plasma is confined momentarily between the surface of the part and the transparent overlay, a rapidly rising high-pressure shock wave is created, which propagates into the part. When the peak stress created by the shock wave is above the dynamic yield strength of the metal, the metal yields, and the metal is “cold worked” or plastically deformed on, and just under, the surface. This plastic deformation results in compressive residual stresses in the surface of the part. The depth and magnitude of the residual stresses depend upon the magnitude and rate of attenuation of the shock wave as it passes through the surface layer, and upon the material properties and the processing conditions specific to the application. Compressive residual stresses typically extend as deep as about 0.040 to about 0.060 inches (about 1.0 to about 1.5 mm) into the surface and can approach the yield strength of the material.
Laser shock peening has been particularly effective at preventing fatigue failures in aircraft engine metal alloy fan and compressor blades. However, the potential application of this process is much broader. The application can encompass aerospace structures, helicopter gears and propulsion components, automotive parts, orthopedic implants, tooling and dies, and numerous other military and industrial components prone to metal fatigue failures.
In some circumstances, particularly those involving metal materials having very thin sections and alloys having low ductility, the high intensity stress waves created during laser shock peening have the potential to cause microstructural damage to the parts being processed. In thin section materials, the compressive stress wave reflects back from the opposite free surface of the part as a relatively high tensile stress wave. In cases where the magnitude of the reflected tensile stress wave exceeds the tensile strength of the material, micro-scale defects can occur. Such damage may include small “microcracks” (typically less than about 0.020 inches in length) or small grain boundary separations between microstructural phases, which tend to be the weakest regions in the material.
The present embodiments disclose methods, systems, and apparatuses for laser shock peening metal materials, including thin metal materials, while preventing or reducing the occurrence of laser shock peening-induced micro-scale defects.